Large area, i.e. two feet or more, by eight feet or more thermoplastic panel-type products are very difficult to make by conventional molding techniques. The present invention enables such large area products to be made even extending up to twenty or forty feet long, or longer, if desired, plastic panel-type products having three-dimensional patterns and surface textures on one or both faces that may be similar or dissimilar with or without a continuously changing profile and with or without shaped edges, such as shake shingle panels, rough board and batten siding panels, rough barn board siding and wall panels, wainscoting panels, brick panels, stone panels, patterned door panels, similar structural wall, roofing, siding and door panels, ribbed soffits, fascia board, flooring reinforced with egg crate ribs, forms for pouring contoured concrete structures and other products having three-dimensional patterns and surface texture. Insofar as we are aware, the prior art does not contemplate, nor suggest, the manufacture of such large three-dimensional patterned and surface textured panels and does not teach those skilled in the art how one should proceed to solve the many problems involved in making such large area products rapidly and economically, so as to be widely available to consumers, builders, and home remodelers at a competitive cost for practical, everyday utility.
The present invention provides a continuous forming process and apparatus for making such large area panel-type products and other three-dimensional pattern and surface textured products from thermoplastic material, providing a very large volume of such products per unit time. By virtue of this large productivity afforded by the present invention, the capital facility investment and tooling costs of the apparatus, as reflected in the cost of each product being produced, are advantageously minimized while achieving three-dimensional patterned and surface textured products.
The current status of the prior art relating to the processing of plastic materials into products is described in the 1974-1975 Edition of Modern Plastics Encyclopedia, Volume 51, No. 10-A, dated October 1974, as published by McGraw-Hill, Inc. On pages 2 and 3 of this Encyclopedia appears a complete index which includes a section relating to the processing of plastic materials into products. This Index section, entitled "Primary processing" appears as set below, in which the numbers refer to pages in this Encyclopedia.
______________________________________ Extrusion 352 Primary processing 277 Foam processing 366 Blow molding 278 Expandable PS molding 366 Extrusion blow molding 278 Extruded thermoplastics foam 371 Injection blow molding 284 Structural foam melt methods 379 Calendering 288 Urethane foam processing 377 Casting of thermoplastics 291 Injection molding 384 Casting of acrylics 291 Laminating of film 410 Casting of nylon 298 Mechanical forming 414 Casting of PP film 298 Blanking 414 Casting of PVC film 301 Forging and solid phase forming 416 Casting of thermosets 301 Plastisol processing 423 Centrifugal molding of RP Radiation processing 419 304 Coating 308 Reinforced plastics/composite Extrusion coating and processing 427 laminating 308 Filament winding 427 Melt roll coating 322 Low-pressure molding 438 Powder coating 328 Matched die molding 433 Transfer coating 330 Pultrusion 444 Compression molding 332 Rotational molding 446 Controls and instrumentation Testing equipment 463 335 Extrusion controls and Thermoforming 451 instrumentation 335 Tooling 470 Injection controls and Dies 470 instrumentation 338 Injection mold 473 Transfer molding 456 Web impregnation 460 ______________________________________
This description of the prior art in Modern Plastics Encyclopedia is incorporated herein by reference as background to show the actual current practices throughout the plastics industry. Certain processes of the prior art are briefly described hereinafter.
Injection molding is a batch process which can be used to produce plastic products having three-dimensional shapes, but the sizes of articles which can be made and the speed of the production cycle are now severely limited by practical and economic factors. The molds must be very rigid and hence massive to resist distortion from the high pressures under which the injected plastic is caused to flow throughout the mold. Thus, heavy costly equipment is needed to move these massive molds. There is a limitation to the practical size of injection molds, because of the clamp forces necessary to hold the molds closed, which already are of the order of 1,000 to 3,000 tons for the largest conventional injection molds. The plastic material is heated up to a high temperature level within its plastic range, so that it will flow in the mold. Often, the plastic is heated to a temperature of about 600.degree. F., which includes a substantial superheat. The hot plastic is forced into the mold under great pressure, for example 20,000 pounds per square inch. When the hot material has flowed in and throughout to fill all of the mold cavities, the massive molds, together with the hot plastic therein, must be cooled requiring that a relatively great amount of heat energy (Btu's) be removed. This cooling is accomplished by running cold water through numerous passages within the mold wall.
A lengthy time period occurs while the running water cools the mold so as to cool the plastic material, and the removal of the heat energy from the superheated plastic represents a waste of thermal energy. Finally, the mold is opened, the injection molded articles are removed, and the cycle is repeated.
In order to make injection molding economical, a number of small similar articles are often molded simultaneously during each cycle in massive molds, sometimes weighing up to thirty tons.
In addition to the lengthy time cycle and the costly, massive molding equipment often used, injection molding consumes a large amount of thermal energy which must be withdrawn and rejected in each cycle by the cooling water.
The extrusion process can be used to produce plastic articles of a continuous cross-sectional profile having no variation in the longitudinal plane. Extrusion can be a relatively high speed production technique, i.e. a high volume per unit time can be produced. Tooling costs associated with extrusion forming of plastic materials are usually far less than those for injection molding, but there is the limitation that the cross-sectional profile of the extruded product remains fixed. Sometimes extruded products are formed with very simple variations in cross section or laterally such as by corrugations, but essentially the wall thickness and cross section of the extrusion remains fixed, as determined by the shape of the extrusion nozzle orifice. For example, U.S. Pat. No. 3,751,541 -- Hegler shows the continuous formation of partially transversely corrugated thermoplastic tubing by extruding the tubing and passing the tubing while still in the thermoplastic state between two sets of revolving semi-circular corrugated mold half assemblies, each set of mold halves being connected together in endless fashion and applying a vacuum to the outside of the tubing between the corrugated mold halves to draw the tubing wall against the corrugated mold halves. U.S. Pat. No. 3,864,446 -- Maroschak is similar to Hegler except that short inserts are positioned in the semi-circular valleys of the semi-tubular mold sections for producing interrupting gaps or recesses in certain annular ribs of the corrugated pipe being molded. These tubing processes cannot produce panel-type products having three-dimensional pattern and surface textures on one or both faces that may be similar or dissimilar and which may be one inch or more thick and in which the profile can vary in thickness across the width of the product by one inch or more, because the product is formed by applying vacuum to the outside necessarily causing the inner surface of the tubing to be generally the negative image of the outer surface.
In the thermoforming process, a previously made rigid sheet of thermoplastic material of uniform thickness is placed adjacent to a forming die. While the sheet is heated, a vacuum may be created between the die and the sheet. Atmospheric pressure, or other fluid pressure source, then pushes the sheet against the die into conformance with the die shape. The thermoforming process produces a three-dimensional pattern on one face of the article, but the back surface of the article is generally a negative image of the front face with somewhat rounded contours. The thickness of the plastic sheet material used for thermoforming is usually limited to less than one-quarter of an inch because of the need for this sheet material to become accurately conformed to the die face by the fluid pressure being exerted. Panel-type products having dissimilar three-dimensional patterns and surface textures on both surfaces cannot be thermoformed nor can the thicknesses and thickness variations, as discussed, be obtained. Thermoforming is in essence not a truly continuous process, and the length of the finished product is limited to the length of the initial sheet.
Thermosetting materials can be formed by compression molding by placing the material between dies where it is subjected to heat and pressure. But the size of the articles to be made is limited by the magnitude of the forces involved in the compression operation, and the production rate is limited by the time required for the thermoset to take effect throughout the product. The concept and procedural steps involved in such compressive thermosetting molding are quite different from the continuous extrusion and casting steps of the present invention providing the numerous advantages summarized below.
Various types of articles are produced by introducing expandable plastic granules into a mold cavity where they become coherent and are formed by the expansion pressure of the materials, for example articles made of foamed polystyrene. This type of molding is generally called foam processing. Continuous foam processing is shown in Yovanovich -- U.S. Pat. No. 3,736,081 wherein granules of thermoplastic material containing a heat expanding agent are continuously fed into a travelling mold channel defined by attachments on a chain belt and then they are heated by steam introduced under pressure into the mold channel. In Hall -- U.S. Pat. No. 3,888,608 -- prefoamed polymer particles are fed into a channel defined between endless belts composed of transverse strips of alternatively gas-permeable and gas-impermeable material and are heated by steam passing through the perforated belts causing further foaming to take place for fusing the particles together. Such foam processing in which the shaping of the product depends upon the internal foaming pressure of the expanding material is quite different from the continuous ribbon feeding and shaping steps of the present invention in which the final shaping of the product is produced by mechanically rolling continuous travelling flexible cooled molds against opposite surfaces of the thermoplastic material.
In U.S. Pat. Nos. 3,764,642 and 3,879,505, of Boutillier are described methods of making extruded profiled sections of expanded thermoplastic material having an integral thick skin. The profile of the extrudate is constant along the length of the product, because the extrudate is pulled through a stationary cooled shaper of constant cross section by a drawing caterpillar. Forming is obtained by foaming expansion pressure against the stationary walls of the shaper and not by mechanically impressing continuous travelling molds against opposite surfaces of material. These methods of the patentee cannot make panel-type products having three-dimensional patterns and surface textures on one or both surfaces, nor those products in which the surface profiles change along the length of the product.
In Di Benedetto, et al. -- U.S. Pat. No. 3,841,390 multiple metal pieces are cast in a plurality of mold cavities by flowing molten metal downwardly through a pouring chute into a runner channel located between the upper edges of a pair of closed loop belts made of flexible material, such as vulcanized rubber. These rubber belts revolve in the horizontal plane around a plurality of pressure plates carried on the outer surface of a revolving sprocket chain. A line of individual metal castings is formed in which the individual articles are connected to a runner casting by sprue portions from which they are broken away. The impressions are formed by gravity flow of molten metal down into a pre-existing cavity already defined by the rubber belts which previously had been pressed together in face-to-face contact. This molten metal casting action is quite different from initially impressing revolving cooling molds onto opposite sides of a ribbon of heated thermoplastic material by progressive localized rolling pinching action to change the thermoplastic ribbon into the specific profile shapes and textured patterns desired and then holding the traveling molds against the impressed material to retain the impression while the material is cooled into its memory retention state.
The reader may be interested in Gartaganis, et al. -- U.S. Pat. No. 3,712,843, wherein corrugated board is made by applying heat to the newly assembled corrugated board by the planar surface of an endless moving steel belt. In this laminating of corrugated board, the steel belt is heated, not cooled. In Smith -- U.S. Pat. No. 3,726,951 walled structures are made of foam-in-place plastic material deposited by a progressively moving foaming head having multiple driven planar belts forming a channel into which the foaming plastic material is passed as the head progresses about the structure. See also U.S. Pat. No. 3,837,774 -- Ross, et al. in which elongated rigid molded products, such as strip molding for trimming door and window frames, are continuously produced from curable resin by mixing and pouring a curable liquid or foam-type synthetic material from nozzles onto a lower revolving mold belt just before it mates with an upper revolving mold belt to define a closed moving cavity of constant cross section. The mold belts are shown being heated to cure the foam material to form the product having an inner core of lesser density and a skin of greater density. In U.S. Pat. No. 3,837,781 -- Lambertus multiple side-by-side strands of synthetic plastic material are extruded and delivered into the V-shaped grooves of a multi-groove cooled conveyor band. Each strand lies in one lengthwise groove of the band and is carried along by the frictional adhesion and a roller presses the strands into the grooves to prevent detachment of the strands from the groove. In none of these four patents (Gartaganis, et al., Smith, Ross, et al. nor Lambertus) is there a ribbon of heated thermoplastic material against whose opposite sides two cooled revolving flexible belt molds are rolled under pressure by opposed nip rolls to impress three-dimensional patterns and textured surfaces thereon, the travelling belt molds being thereafter held under pressure against opposite sides of the impressed thermoplastic material to cause it to retain its shape while being cooled by the belt molds into its memory retention state.
The reader may be interested in the application of flexible steel belts to the casting of molten metal in smooth strips, billets or slabs, as described in Hazelett U.S. Pat. Nos. 2,904,860; 3,036,348; 3,041,686; 3,123,874; 3,142,873; 3,167,830; 3,228,072; 3,310,849; 3,828,841; 3,848,658 and 3,878,883.